1. Field of the Invention
The present invention relates to a semiconductor laser recording apparatus and more particularly to video data signal processing suitable for recording images of high resolution and high quality.
2. Description of the Related Art
As described in JP-A-61-234168, this type of recording apparatus uses a pulsed laser turn-on signal transmitted from an information processor to the recording apparatus and when dots which are continuous in the main scanning direction are to be recorded, the pulse signal assumes a 100% duty continuing over the continuous dots. Accordingly, a beam spot shape formed on the recording medium by a laser beam is longitudinally or in a direction perpendicular to the scanning direction elongated and when the spot size in the scanning direction is not less than about 1/2 of the dot pitch, the contrast potential, i.e. a difference between the potential of electrostatic charge representing a latent image on a photosensitive element and the bias potential used in development of the latent image, in development of an image produced by one dot irradiation is lower than that of an image produced by two or more dots irradiation. Therefore, in a known recording apparatus, when one isolated dot is recorded, current applied to the laser diode is increased, thereby intensifying the laser beam for increasing the contrast potential and improving the picture quality.
However, the method of instantaneously increasing the current applied to the laser diode involves the following problems:
(1) Two constant current switching circuits for high-speed operation are required to be used for the laser beams of different intensities.
(2) The time required for adjusting deviation in slope efficiency is doubled.
(3) In the semiconductor laser, the optical output is highly sensitive to the drive current and a slight change in the current leads to a large change in the optical output. Therefore, the laser beam output is liable to fluctuate in response to deviation in temperature and adjusted value between the two switching circuits.
FIGS. 6A to 6E show how a video data signal or video data 1 is related to the laser beam energy distribution and the recording dot shape. The video data 1 in FIG. 6A indicates one signal for isolated one-dot recording and another signal for continuous three-dot recording.
When the video data 1 assumes a high level in the isolated one-dot recording signal, the semiconductor laser diode is turned on and a spot 1 is formed by a laser beam on a photoelectric recording medium for electrophotographic recording. The laser beam is deflected by a deflection mirror to move the laser spot 1 in a direction of deflection as shown in FIG. 6B. An optical energy distribution characteristic applied by the laser spot 1 to the photoelectric recording medium has a component as shown by curve 2 in FIG. 6C in a moving direction x of the laser spot 1 and a component as shown by curve 3 in FIG. 6D in a direction y orthogonal to the moving direction. The photoelectric recording medium sensitized in accordance with the distribution characteristic is developed to obtain a recording dot which has an isolated one-dot shape indicated at reference character 4a in FIG. 6E when the threshold level is at Eth. Similarly, the continuous three-dot recording signal presents a continuous three-dot shape indicated at reference character 4b in FIG. 6E.
But the recording dot shape, especially the isolated one-dot shape 4a is unbalanced in the ratio between longitudinal dimension x1 and lateral dimension y1 to provide a longitudinally elongated dot shape which makes a bad appearance. In order to correct this disadvantage, as shown in FIGS. 7A to 7E, the video data is cut away at its leading edge to provide a recording dot 4a' having a longitudinal dimension x1' and a lateral dimension y1 nearly equal to x1'. With the video data cut away at its leading edge, the optical energy acting on the photoelectric recording medium is decreased as shown at curve 5 and hence, in order to obtain an elevated curve 6, the drive current for the semiconductor laser diode must be increased for recording the isolated one dot.